This invention relates generally to the supply of electrical current to a telephone subscriber loop and more particularly to the supply of line current to a telephone loop employing a fixed reference voltage to accommodate a wide variety of different subscriber requirements.
The prior art contains a number of patents descriptive of circuitry which circuitry supplies line current to a telephone subscriber loop. Essentially, various circuits described in prior art patents serve to maintain a constant or an efficient line current in a telephone subscriber loop, the length of which may vary, and, therefore, the resistance of the line may vary depending upon the distance from the subscriber to the central office. The line feed function associated with a classic telephone circuit is equivalent to two resistors each in series with one terminal of the central office battery.
The value of the battery is in the range of 48 to 60 volts, and the series resistors typically vary between 200 to 11K ohms or greater depending upon the particular telephone exchange. The line feed resistors are selected at the central office of a value necessary to produce the proper line current versus loop resistance characteristics as expected by the telephone instrument. As is understood, a long telephone line indicates a larger resistance, while a short line has a lower resistance. The line feed resistors are placed in series with the telephone line and selected according to the desired loop current characteristics.
As is known, these resistors dissipate power and may create thermal problems. As above indicated, certain of the systems of the prior art attempt to circumvent the power dissipation problem by providing a constant current to a telephone line which is relatively independent of the length of the line.
For examples of suitable systems reference is made to U.S. Pat. No. 4,315,106 entitled APPARATUS FOR REGULATING CURRENT SUPPLIED TO A TELEPHONE LINE SIGNAL OF THE TYPE EMPLOYED IN DIGITAL TELEPHONE SYSTEMS by R. C. W. Chea., Jr. and assigned to International Telephone and Telegraph Corporation, the assignee herein. Another pertinent patent in U.S. Pat. No. 4,317,963 entitled SUBSCRIBER LINE INTERFACE CIRCUIT UTILIZING IMPEDANCE SYNTHESIZER AND SHARED VOLTAGE SOURCE FOR LOOP CURRENT REGULATION CONTROL by R. C. W. Chea, Jr. and assigned to International Telephone and Telegraph Corporation, the assignee herein and issued on Mar. 2, 1982. A third patent of interest in U.S. Pat. No. 4,387,273 entitled "SUBSCRIBER LINE INTERFACE CIRCUIT WITH IMPEDANCE SYNTHESIZER" by R. C. W. Chea, Jr. and assigned to International Telephone And Telegraph Corporation, the assignee herein.
Essentially, the objects of the above noted patents are to supply a current to a telephone subscriber line from a given voltage source and to maintain that current at an efficient value so as to reduce dissipation in the line circuit and to overall reduce the complexity of the line circuit. A major reason for reducing complexity and power dissipation in a line circuit is inherent with the fact that the line circuit in a telephone is associated with each subscriber.
Hence the number of line circuits in a telephone system is equal to the number of subscribers and, as one can ascertain, can be an extremely large number in the case of a large switching system. An example of an extremely successful telephone system is designated as the ITT 1240 System. This system is employed throughout the world and exists as the major switching system in many countries. The system is extremely popular, and there is a great deal of information concerning the operation of the system in the patent literature as well as in technical periodicals. For example, see an article entitled "ITT 1240 Digital Exchange Switching Network" by J. M. Cotton et al, published in ELECTRICAL COMMUNICATION, Volume 56, 1981.
Essentially, the technique used in supplying line current to a subscriber telephone line or a trunk is to change a reference voltage in the line circuit to accommodate the change in central office battery and thereafter to change the gain of the impedance synthesis loop to accommodate the change in the series resistances which as indicated are placed in series with the central office battery. This technique is applied for each of the required feed characteristics of the various systems employed.
For an example of such a technique, reference is made to U.S. Pat. No. 4,317,963 as indicated above. Based on the above, it is again noted that traditional line feed characteristics used in telephone systems are derived from a constant voltage which is the central office "talk" battery in series with a pair of equal valued resistors or feed resistors which serve to maintain longitudinal balance. Accordingly, many telephone sets are equalized for line length variation using this line feed assumption. Certain government authorities such as the British Post Office have agreed to use constant current to save power, but they consequently require gain compensation in the line circuit to allow for the loss of equalization which is built into the constant voltage telephone set.
It is thus an object of the present invention to provide a circuit for supplying line current to a subscriber line which circuit affords low power dissipation and which eliminates the need for constant current and the consequent gain control as utilized in the prior art.
A further object of the present invention is to provide a circuit for supplying line current which requires no changes in loop gain to accommodate the changes in feed requirements based on different system characteristics.